Inserts for metal cutting purposes

ABSTRACT

The present invention relates to a cutting tool with increased tool life when machining grey cast iron. The tool comprises a solid CBN-containing body containing more than 60 vol % CBN and a coating. The coating consists of at least one layer of metal nitride, carbonitride or carbide with the metal elements selected from at least one of Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al, with a total thickness of 2-7 μm and 2-8 μm of α- and/or κ-Al 2 O 3 . The coating is a CVD coating deposited at 900° C. or higher.

This application is a national stage application of PCT/SE00/00886 under35 U.S.C. §371, filed May 4, 2000, that designated the United States ofAmerica, and which claims priority to Swedish Application No. 9901649-5,filed on May 6, 1999, and which PCT application was published in Englishon Nov. 16, 2000.

FIELD OF THE INVENTION

The present invention relates to a cutting tool with increased tool lifewhen machining grey cast iron.

BACKGROUND OF THE INVENTION

In the description of the background of the present invention thatfollows reference is made to certain structures and methods, however,such references should not necessarily be construed as an admission thatthese structures and methods qualify as prior art under the applicablestatutory provisions. Applicants reserve the right to demonstrate thatany of the referenced subject matter does not constitute prior art withregard to the present invention.

Cutting tools having cutting edges formed of a superhard abrasive suchas a cubic boron nitride (CBN) based material are manufactured by powdermetallurgical techniques and are to an increasing extent used for themachining of cast iron and hardened steel. Several types of CBN cuttingtools are known, the majority consisting of a CBN tip that has beenbrazed onto a cemented carbide insert. Others have the CBN sintereddirectly to a cemented carbide backing of sufficient thickness toproduce an insert while yet others consist of mainly CBN without anycemented carbide backing.

Subjecting an insert with a brazed on CBN/cemented carbide tip totemperatures over 700° C. will result in destroying the braze, but forinserts without any braze joint there is no such limitation.

When machining hardened steels, e.g., ball bearing steels, chemicaldissolution of the tool material is an important part of the wearmechanism, provided that the tool material has sufficient hot hardnessto resist plastic deformation. For that reason the CBN materials usedfor such work materials usually contain a high fraction of secondaryphases with low chemical solubility in steel. In WO 98/28464 it is shownin the example that applying a coating including a MTCVD-TiCN and aCVD-Al₂O₃-layer to such a low CBN tool material, substantial advantagesin tool life can be achieved when machining hardened ball bearing steel.The CVD-coating is applied to a substrate comprising cemented carbideand at least one CBN body whereby the cemented carbide constitutes themain part of the substrate.

When machining grey cast iron, the dominating wear mechanism is ofabrasive nature, and consequently due to the high hardness of CBN, themost suitable material for grey cast iron is that of high CBN content,generally more than 60%, often over 80%. An example of such a materialis disclosed in U.S. Pat. No. 5,326,380. Inserts of this type areusually produced as a layer of CBN-containing material bonded throughsintering to a cemented carbide backing, or a solid CBN-compact thushaving no braze joint on the finished insert that can be damaged atnormal CVD coating temperatures.

SUMMARY OF THE INVENTION

The tool comprises a CBN-containing body with more than 60 vol % CBN anda coating. The coating consists of at least one layer of metal nitride,carbonitride or carbide with the metal elements selected from at leastone of Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al and at least one layer ofAl₂O₃. The coating is a CVD coating deposited at 900° C. or higher.

DETAILED DESCRIPTION OF THE INVENTION

Now it has been found that applying a CVD-coating to such an insert madefrom high CBN-content material will also increase the performanceleading to increased tool life when machining grey cast iron.

In one preferred embodiment the insert is of solid CBN i.e. withoutcemented carbide support.

In another preferred embodiment the insert is provided with a centralhole for clamping.

In a most preferred embodiment the insert is of solid CBN with a centralhole for clamping. This central hole is made by laser machining.

The coating can consist of at least one layer of a metal carbide,nitride and/or carbonitride with the metal element selected from atleast one of Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al, preferably TiC,TiCN, TiN or TiAIN plus at least one layer of Al₂O₃. The Al₂O₃-layer canbe α and/or κ phase. The total coating thickness of the nitride,carbonitride or carbide layer can be 2-7 μm, preferably 3-5 μm, and thetotal thickness of Al₂O₃ can be 2-8 μm with a preferred thickness of 4-7μm.

The total coating thickness according to the invention varies between 4and 15 μm, preferably between 5 and 10 μm.

The number and thickness of individual coatings can be varied, but thetotal accumulated thickness of the individual coatings falls within thelimitations stated above.

A further improvement in cutting performance can be anticipated if theedges of the Al₂O₃ coated cutting tools according to the invention aretreated by a gentle wetblasting process or by edge brushing with brushesbased on e.g. SiC as disclosed in U.S. Pat. No. 5,861,210.

EXAMPLE

A Sandvik CBN insert SNGN 120408F in grade CB50 containing about 90% CBNby volume was coated with a 4 μm inner layer of TICN and it 6 μm outerlayer of α-Al₂O₃ using CVD-technique at a temperature of about 1000° C.

It was then used to machine workpieces of pearlitic grey cast iron(Swedish standard SS0125) at a cutting speed of 500 m/min using a feedrate of 0.3 nm/rev and a depth of cut of 0.5 mm. The table below showsflank wear in mm versus time in cut under dry cutting conditions. Anuncoated CBN of the same composition was used as a reference.

Number of passes Variant 4 8 12 16 20 24 28 Uncoated 0.09 0.11 0.21 0.260.30 0.34 0.35 reference Present 0.05 0.07 0.09 0.09 0.10 0.12 0.13invention

While the present invention has been described by reference to theabove-mentioned embodiments, certain modifications and variations willbe evident to those of ordinary skill in the art. Therefore, the presentinvention is to limited only by the scope and spirit of the appendedclaims.

What is claimed is:
 1. A cutting tool for the machining of grey castiron comprising: a CBN-substrate comprising more than 60 vol % CBN; anda CVD coating deposited at 1000° C. or higher comprising at least onelayer of metal nitride, carbonitride or carbide with the metal elementsselected from the group consisting of Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, Wand Al and at least one layer of α and/or κ-Al₂O₃, and wherein a totalthickness of the at least one layer of metal nitride, carbonitride orcarbide is 2-7 μm and a total thickness of the at least one layer of α-and/or κ-Al₂O₃ is 2-8 μm.
 2. The cutting tool according to claim 1,wherein the cutting tool has a central clamping hole.
 3. The cuttingtool according to claim 1, wherein the substrate has a backing that iscemented carbide free.
 4. The cutting tool according to claim 1, whereinthe total thickness of the at least one layer of metal nitride,carbonitride or carbide is 3-5 μm.
 5. The cutting tool according toclaim 1, wherein the at least one layer of metal nitride, carbonitrideor carbide is a layer of TiC, TiCN, TiN or TiAIN.
 6. The cutting toolaccording to claim 1, wherein the total thickness of the at least onelayer of α- and/or κ-Al₂O₃ is 4-7 μm.
 7. The cutting tool according toclaim 1, wherein the substrate comprises at least approximately 90volume % CBN.
 8. A method of machining a grey cast iron, comprising:providing a cutting tool comprising a solid CBN-substrate comprisingmore than 60% by volume CBN and a CVD coating deposited by CVD at atemperature of 1000° C. or higher, said coating comprising at least onelayer of metal nitride, carbonitride or carbide having at least onemetal element selected from the group consisting of Ti, Nb, Hf, V, Ta,Mo, Zr, Cr, W and Al, and at least one layer of α- and/or κ-Al₂O₃,wherein a total thickness of the at least one layer of metal nitride,carbonitride or carbide is 2-7 μm and a total thickness of the at leastone layer of α- and/or κ-Al₂O₃ is 2-8 μm; and machining the grey castiron with the cutting tool.